Previously, researchers believed that marine mammals could be so large because the buoyancy of water frees them from the constraints of gravity. Although this freedom may still be a factor, Gearty says that his results show that marine mammals need their heft to keep themselves warm in the often chilly oceans.

“These animals are big for very specific reasons. It’s not that they could be big, it’s that they must be big,” he says.

BIGGER IS BETTER?

When Gearty and colleagues created a series of computer models analyzing factors that influence size, they found two that converged to determine body size in aquatic mammals.